THE PHYSICAL GEOGRAPHY OF THE PAST. 
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geologists are agreed, but on the question as to how that 
lateral thrust has been brought about there are at least two 
distinct opinions. 
The hypothesis, which has up to recently found most 
favour with geologists, and which taken by itself perhaps 
explains the greatest number of observed facts, is the so-called 
hypothesis of Secular Contraction. 
That the earth was originally a molten mass, which has 
gradually cooled from within outwards, is rendered highly 
probable from certain astronomical considerations; and that 
its interior has still a very high temperature is indicated 
both by volcanic phenomena, and by the fact that the deeper we 
go down the hotter it becomes; the increment of temperature 
being about 1° Fahr. for every fifty feet of depth. The hot 
interior or nucleus must still be cooling down by conduction 
of its heat through the solidified crust and its dissipation into 
space, and this cooling must also mean contraction. There is, 
consequently, a constant tendency for the interior nucleus to 
separate itself from the outer and cooler shell, and since it is 
manifest that the shell cannot stand alone, it must tend by 
the power of gravitation to adapt itself to the “ diminishing 
circumference of the contracting interior,” and in its efforts 
to do so, great lateral pressure is evoked, which bends, breaks, 
and ridges the crust along certain lines. Thus, on this 
hypothesis, have been produced the great lines of elevation 
of most of our mountain ranges. They may not inaptly be 
compared with the wrinkles on the skin of a drying apple, 
for the skin of the apple becomes wrinkled in its efforts to 
adapt itself to the shrinking interior of the fruit. 
We should certainly expect that the elevation of ridges on 
the earth’s surface, if these are to be looked upon as the 
expression of secular contraction, would take place along the 
lines of least resistance, where, in fact, the earth’s crust is 
the thinnest; it is, consequently, a little startling at first to 
find that the great elevations have nearly all taken place 
where sedimentation has been the thickest, and where we 
might expect the crust of the earth to be the strongest. The 
difficulty, however, disappears on examination, and for the 
explanation we will return once more to the Carboniferous 
strata of the Pennine area. These, as we have seen, were 
laid down in a great trough which gradually bent more and 
more downwards as more sediment accumulated in it. Such 
a great and constantly deepening depression in the earth’s 
surface is called a geosynclinal, and in such troughs have been 
deposited, sometimes to a thickness of miles, the strata which 
are now elevated in our mountain chains. 
